1. Field of the Invention
This invention relates generally to pulse tube cryocoolers and more particularly to a structure that can be substituted for the reservoir that is used in common configurations and thereby reduce cost, working gas volume, weight and cool down time.
2. Description of the Related Art
Traveling wave pulse tube coolers have been recognized as having desirable characteristics for cooling to cryogenic temperatures, particularly when multiple coolers are cascaded in stages. Their development began with the study of the cooling effects resulting from the application of a pressure wave to one end of a tube that was closed at its opposite end. A regenerator was added to the tube and an example is illustrated in U.S. Pat. No. 3,237,421. The art recognized that the time phasing between the pressure and the working gas mass flow velocity in the regenerator was critical to the heat pumping efficiency of the cooler. A dramatic improvement in performance resulted from the addition of an orifice, at the formerly closed end of the tube, with the orifice leading to a relatively large volume reservoir, also referred to as a surge volume, compliance volume or buffer. This orifice pulse tube cooler greatly improved the phasing in the regenerator thereby increasing heat pumping efficiency. Numerous examples of the orifice pulse tube cooler exist in the prior art of which U.S. Pat. No. 5,794,450 is only one example.
The orifice and reservoir changed the acoustic impedance at the end of the tube and thereby changed the phase relationship between gas velocity and pressure. At the wall of a closed end of a tube, the boundary condition velocity is always zero while the pressure oscillates and therefore the closed end has a pressure anti-node and a velocity node. The closed end presents a nearly pure reactive impedance to the tube, with the pressure and velocity essentially 90° out of phase and reflecting energy. An orifice, however, when connected to a large volume, that is sufficiently large that it does not undergo any significant pressure variation, allows gas to flow in oscillating directions through the orifice unaffected by a pressure change in the reservoir (because there is none) and allows pressure variations across the orifice, if the orifice is not too large. Consequently, the combined orifice and reservoir can be designed to present a resistive acoustic impedance to the tube. The resistive impedance has the characteristic that the pressure and velocity of the gas at the orifice are in phase. The phasing change at the end of the tube resulting from substitution of the orifice and reservoir for the closed end wall resulted in a desired change in the phasing in the reservoir ultimately resulting in the improved heat pumping efficiency.
Pulse tube coolers have also been configured with multiple cascaded stages as illustrated in U.S. Pat. No. 6,256,998 and U.S. Pub. 2004/0000149.
The traveling wave pulse tube cooler was further improved by substitution of an inertance tube for the orifice. An example of this configuration is illustrated in U.S. Pub. 2003/0226364. The inertance tube is a long narrow tube, typically a few meters long, that is open at each end and can be wound in a coil. The inertance tube is connected between, and inserts a reactive acoustic impedance between, the reservoir and the pulse tube. When connected in this manner to the pulse tube and cut to approximately ¼ wavelength of the acoustic wave, this combination presents a nearly resistive acoustic impedance to the end of the pulse tube. Using an inertance tube instead of an orifice, a designer can, by varying the length of the inertance tube, vary the acoustic impedance, and therefore the pressure/velocity phasing, at the end of the pulse tube. This permits the designer more flexibility to further adjust and optimize the phasing in the regenerator and thereby further increase the heat pumping efficiency.
The reservoir, however, also has some undesirable characteristics. The reservoir must enclose a large volume that is sufficiently large that the pressure of the gas within it does not vary appreciably throughout an acoustic cycle. Furthermore, the reservoir must be sufficiently strong that it will retain the working gas under the average pressure to which the pulse tube cooler is charged. Therefore, the reservoir must be structurally configured and have both its surface area and its thickness sufficiently large to meet these requirements. As a consequence the reservoir has a large mass, has a large volume occupying considerable space, is relatively heavy and is relatively expensive to manufacture.
Additionally, in multi-stage pulse tube cryocoolers, the upper stages (stages beyond the first stage) operate in their steady state at reduced temperatures. In some implementations, the reservoir and inertance tube for an upper stage operates at the temperature of its warm region or “end” which is at the temperature of the cold region or “end” of the preceding stage. Therefore, under transient conditions when the cryocooler is cooling down to its operating temperature, the pulse tube cooler stages must cool down the reservoir as well as other components. The relatively large mass of the reservoir, and its consequent high heat storage capacity, causes a substantial time delay until the cryocooler reaches operating temperature.
It is therefore an object and feature of the invention to substitute for the reservoir of a pulse tube cooler, a structure having a greatly reduced mass and volume that is also considerably less expensive and easily made from a readily available, common product, and can be more easily contained within the outer vacuum vessel in which cryocoolers are ordinarily housed.